1. Field of the Invention
This invention relates to photographic lenses of the partial focusing type having a high relative aperture and a widened image angle while still permitting the total length of the lens system to be reduced to compactness, and more particularly to 4-component photographic lenses of the partial focusing type.
2. Description of the Prior Art
Recently, as the trend of reducing the size of photographic cameras prevails, there is an increasing demand for compact photographic lenses of short total length. In order to design a lens having a smaller ratio of the distance from the front vertex to the focal plane to the focal length (or smaller telephoto ratio) than unity, it is desirable to employ the lens type that the front assembly is of positive power and the rear assembly is of negative power, or the so-called telephoto type. The telephoto type of lens is, however, not much suited to increase the field angle. Another recent trend is that the auto-focus cameras are finding wider acceptance in various fields of activity. To achieve accurate and quick and easy automatic focusings, it is preferred to select a lens group which is small in the number of members and light in weight for employment in focusing.
Under such situations, it is now strongly required to make the focusing provision at part of the lens system which allows for the employment of the telephoto type to achieve a minimization of the bulk and size of the lens sytem and an increase of the angular field coverage.
In the past, such partial focusing method employed in the photographic lens was by moving the first three lens components counting from the front as a unit as proposed in Japanese Laid-Open Patent No. Sho 56-91206.
The movement of the 3 lens components in unison, however, tended to change spherical aberration and field curvature in opposite directions to each other as focusing is performed. In more detail, as focusing is effected down to shorter object distances, the spherical aberration is progressively more under-corrected, while the field curvature is progressively more over-corrected. Upon consideration to a high grade imagery over the entire area of the film frame, such opposition of the directions of change of the spherical aberration and field curvature with focusing cannot be said to be rather preferable.
As other photographic lenses of the partial focusing type mention may be made of U.S. Pat. Nos. 3,695,750 and 3,762,801.